At first, Gulliver is a curiosity to the Lilliputians, being enormous and yet somehow subservient to them (Gulliver has no wish to destroy them, although he is more than able). After teaching Gulliver their language, the Lilliputians draw up a contract of rules and requirements for Gulliver, including the most important:
"6th, He shall be our ally against our enemies in the island of Blefuscu, and do his utmost to destroy their fleet, which is now preparing to invade us."
(Swift, Gulliver's Travels, gutenberg.org)
Although they also request that Gulliver help carry large stones and use his height to bring their messengers great distances, the use of Gulliver as a weapon of war is the most telling. Neither kingdom has had a weapon quite so powerful, and so Gulliver represents the ascension of Lilliput to a position of power. In return, they will feed him and grant him certain freedoms; Gulliver, being a friendly sort, takes advantage of these freedoms to learn more about their society. He is not really excited about the prospect of war, but he does help when it is needed.